The invention is especially concerned with heavy-duty mixers and heavy-duty mixing methods that are capable of use where, for example, the resultant mix has a viscosity of some 3,000 poise (300 N/m.sup.2) or more. Such mixers and mixing methods are applicable, for example, in the mastication and blending of rubbers, in the mixing of rubbers with carbon black and other ingredients for vulcanisation, in incorporating plasticisers and other chemicals into polyvinyl chloride, and in mixing fillers into polyethylene and polystyrene. The known mixers and mixing methods used for such applications have practical shortcomings, and in this regard it is an object of the present invention to provide improved forms of mixer and mixing method.